The present invention relates to radiant electric heaters, such as for use in cooking appliances beneath a cooking surface, such as of glass-ceramic.
It is known to provide a radiant electric heater comprising a dish-like support, such as of metal, having therein a base layer of thermal insulation material, such as microporous thermal insulation material. At least one electric heating element is supported relative to the base layer. Such a heater is well known to be used in cooking appliances, particularly located in contact with the underside of a cooking surface which is commonly of glass-ceramic.
For safety and/or control purposes, it is well known to provide a temperature-responsive device in the heater. Such a temperature-responsive device is known to comprise a rod-like component which typically includes an outer tube of quartz, or of ceramic such as cordierite, and which extends partly across the heater and overlying the heating element. The rod-like component is known to be secured at one end at a peripheral region of the dish-like support and suitably to a switch head which is secured to the edge of the dish-like support. The rod-like component is secured such that it is incapable of articulation at its end where secured and extends in cantilevered manner, without support, partly across the heater.
This arrangement for the temperature-responsive device suffers from a disadvantage in that cooking appliances incorporating the heater are often required to pass mechanical shock load and/or vibration tests, to ensure that damage is unlikely to occur during transportation and operation of such cooking appliances.
A typical requirement is for a cooking hob to pass a drop test in which the hob, packaged for transportation, is dropped onto a solid surface from a typical height of 0.5 to 0.6 metres. During such a drop test, large inertial forces are imposed on the free end of the rod-like component of the temperature-responsive device in the heater. Such forces can cause the free end of the rod-like component to be deflected by a considerable amount. As a result, the rod-like component may fracture, or its free end may strike the underside of the overlying glass-ceramic cooking surface and may cause fracture of the glass-ceramic material of the cooking surface.
Another test carried out by appliance approval authorities simulates a heavy pan being dropped onto a glass-ceramic cooking surface having a heater located in contact with its underside. The resulting shock loading can result in severe deflection of the free end of the rod-like component of the temperature-responsive device in the heater, with undesirable consequences similar to those previously described.
It is an object of the present invention to overcome or minimise this problem.
According to the present invention there is provided a radiant electric heater comprising a dish-like support having a base layer of thermal insulation material; at least one heating element supported relative to the base layer; a temperature-responsive device having a rod-like component secured at one end at a peripheral region of the dish-like support and extending without support partly across the heater over the at least one heating element; and restraining means provided at a free end region of the rod-like component, wherein the restraining means has a first portion thereof secured in the base layer of thermal insulation material and a second portion thereof at least partially surrounding and spaced from the rod-like component to allow limited relative movement between the rod-like component and the second portion of the restraining means, the restraining means serving to limit displacement of the free end of the rod-like component away from and/or towards the base layer of thermal insulation material.
The second portion of the restraining means may be adapted to underlie and/or overlie the rod-like component.
The first portion of the restraining means may comprise one or more legs secured in the base layer of thermal insulation material. A pair of legs may be provided, which may be substantially parallel to one another or arranged with at least portions thereof at an angle to one another, for example such as to effect divergence thereof, and such as where secured in the base layer of thermal insulation material. The legs may have tapered ends to facilitate securing by insertion in the base layer of thermal insulation material.
The restraining means may be of wire form and may comprise wire bent to form the pair of legs of the first portion and necked and/or looped and/or coiled to form the second portion. The wire may be multiply-coiled to form the second portion.
One of the legs may be longer than the other and may pass through an aperture in the dish-like support, such longer leg having an end region bent over behind the dish-like support. The dish-like support may be recessed to accommodate the bent-over end region.
Alternatively the restraining means may be of apertured and/or slotted sheet form, in which case the first portion may alternatively comprise a single leg of plate-like form. Such a single leg may have increased width at a lower edge thereof.
The restraining means may be of apertured and/or slotted metal or ceramic sheet form and may be provided with one or more regions of the second portion arranged at an angle to the first portion and/or with a plurality of regions of the second portion arranged at an angle to one another.
The first portion of the restraining means may be secured by pressing into, or co-moulding with, the base layer of thermal insulation material.
The base layer of thermal insulation material may comprise microporous thermal insulation material.
The first portion of the restraining means may be secured in the base layer of thermal insulation material by embedding to part-thickness or full thickness of the base layer.
The base layer of thermal insulation material may be provided of increased density where the restraining means is secured.
The dish-like support may comprise metal.
The rod-like component may comprise a ceramic material, such as cordierite.
The radiant electric heater may be provided in contact with an underside of a cooking surface, such as of glass-ceramic.
Displacement of the free end of the rod-like component, which is limited by the restraining means, may result from a dynamic mechanical shock load applied to the heater.
For a better understanding of the present invention and to show more clearly how it may be carried into effect reference will now be made, by way of example, to the accompanying drawings in which: